1. Field of the Invention
The present invention relates to a measurement apparatus and a measurement method.
2. Description of the Related Art
Diffused optical tomography (“DOT”) and acousto-optical tomography (“AOT”) are known techniques for measuring a spectroscopic characteristic of a biological tissue. The spectroscopic (or attenuation) characteristic contains an absorption (spectroscopic) characteristic and a scattering (spectroscopic) characteristic, and at least one of the absorption characteristic and the scattering characteristic (which will be also referred to as an “absorption-scattering characteristic” hereinafter) is necessary to measure the biological tissue with a high resolution.
DOT introduces near-infrared light to a test object and detects diffused light, as described in Japanese Patent Laid-Open No. (“JP”)2005-331292. AOT irradiates coherent light and focused ultrasonic waves into the test object, and utilizes an effect of light modulation (acousto-optical effect) due to the interaction between the light and the ultrasonic wave in an ultrasonic focus region (test region), as disclosed in U.S. Pat. No. 6,738,653. JP 2005-331292 assumes the internal distribution of the spectroscopic characteristic and employs an algorithm that is used to modify or reconstruct the assumption in accordance with the measurement result, but the calculation of the internal distribution is complex, an enormous and time-consuming process, and unlikely to converge quickly to the optimal solution. Therefore, U.S. Pat. No. 6,957,096 proposes to partially obtain the spectroscopic characteristic utilizing AOT and to utilize it for the solution.
Since AOT focuses ultrasonic waves in the test region in the test object, the resolution of the test region of AOT is superior to that of DOT because the measurement size of AOT is smaller. However, the light intensity detected by the photodetector in AOT is lower than that in DOT due to focusing. Since the photodetector cannot detect the light unless the light intensity is higher than a threshold, the light intensity becomes too small for AOT measurement as a distance of the test region from the surface of the test object increases. The increased surface area of the focused ultrasonic waves in AOT can overcome the light intensity deficiency, but will result in a deterioration of the resolution. Conventionally, a solution for this problem is not proposed.